Abstract

Given the importance of linear mode properties (e.g., characteristic speeds) in identification/classification of discontinuities, a detailed comparison between the mode properties in fluid theory and kinetic theory in high β plasmas is carried out. It is found that conventional fluid theories of linear modes in both isotropic and anisotropic plasmas do not yield the correct mode properties, even in the long‐wavelength limit. In particular, fluid phase velocities are very sensitive to the model and parameters (polytropic indices) employed. Because of this, fluid theory loses its predictive power. In linear kinetic theory, modes cannot be ordered according to their phase velocities. For instance, at small and moderate propagation angles, the slow/sound (S/SO) mode can have the fastest phase velocity. In such cases, a (quasiparallel) fast shock would be associated with the S/SO mode rather than the usual fast/magnetosonic (F/MS) mode. This has important implications for fast shocks. Since it is the F/MS rather than S/SO mode that connects to the whistler branch, low Mach number quasiparallel shocks associated with S/SO would not be expected to have a phase standing whistler wave train upstream, and their thickness is determined by dissipation rather than dispersion. The consequences of the kinetic modeproperties are demonstrated via hybrid simulations (fluid electron, kinetic ions) using the quasiparallel shock as an example.